Anatomy of the Coccyx (Tailbone)

The coccyx is a triangular arrangement of bone that makes up the very bottom portion of the spine below the sacrum. It is the final segment of the vertebral column and represents a vestigial tail, hence the common term tailbone. 

The coccyx is an integral part of the pelvis, which is vital to essential activities like moving from sitting to standing, walking, eliminating waste, sexual intercourse, and childbirth.

The word coccyx is derived from Greek and means “beak,” as it resembles the shape of a cuckoo bird’s beak when viewed from the side. 

Understanding the Coccyx: Location, Structure, and Fusion

The coccyx comprises 3-5 fused coccygeal vertebrae that get progressively smaller from top to bottom. The entire coccyx measures approximately 4-10 cm in length and is positioned as an inverted triangle—with the base on top and the apex below.¹Protzer L, Seligson D, Doursounian L. The coccyx in clinical medicine. Current Orthopaedic Practice. 2017;28(3):314-317. DOI: 10.1097/BCO.0000000000000502 

When viewed from the side, the coccyx has varying degrees of a convex angle, appearing in the direction of the thoracic spine or middle back. 

The parts of the coccyx are discussed below²Foye PM. Coccydynia: Tailbone Pain. Phys Med Rehabil Clin N Am. 2017;28(3):539-549. doi:10.1016/j.pmr.2017.03.006:

• Base. The base of the coccyx is formed by the first and broadest coccygeal vertebra, Co1, which sits immediately below S5, the 5^th sacral vertebra. Co1 is the only coccygeal vertebra with the following two unique features:
  • Transverse processes: bony attachment points for pelvic soft tissues
  • Coccygeal cornua: two horn-like structures that project upward from the base of the coccyx. The coccygeal cornua articulate with the cornua of the sacrum, forming part of the sacrococcygeal symphysis joint and the intervertebral foramen that the S5 nerve passes through.
• Posterior surface. The coccyx's posterior (back) surface contains small, rounded protrusions that are remnants of the spinous processes of the fused coccygeal vertebrae.
• Anterior surface. The coccyx's anterior (front) surface contains slightly concave grooves that are remnants of the vertebral foramina. 
• Lateral surface. The lateral surfaces or sides of the coccyx serve as points of attachment for nearby soft tissues.
• Apex. The apex of the coccyx is the pointed end, which curves forward and downward. The apex is a point of attachment for soft tissues that support the pelvic floor and anus.

Despite its small size, the coccyx is crucial in providing stability for the lower back and pelvis,¹ the weight-bearing tripod structure that supports a sitting position.

The coccygeal vertebrae fuse in early adulthood

During early development, each vertebra of the coccyx forms independently, similar to the rest of the vertebral column. 

Through adolescence and early adulthood, the vertebrae gradually fuse to form a solid bone structure through a process known as coccygeal ossification.³Woon JTK, Stringer MD. Clinical anatomy of the coccyx: A systematic review. Clinical Anatomy. 2012;25(12):158-167. https://doi.org/10.1002/ca.21216 The timing of coccygeal ossification varies among individuals, but it is typically complete by the mid-20s.³Woon JTK, Stringer MD. Clinical anatomy of the coccyx: A systematic review. Clinical Anatomy. 2012;25(12):158-167. https://doi.org/10.1002/ca.21216

Sacrococcygeal Symphysis: The Articulation between the Sacrum and the Coccyx

The sacrococcygeal symphysis is a small intervertebral joint that connects S5 to Co1.⁴Chaudhry SR, Nahian A, Chaudhry K. Anatomy, Abdomen and Pelvis, Pelvis. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022. https://europepmc.org/article/NBK/nbk482258

Structure of the sacrococcygeal symphysis

The sacrococcygeal symphysis is a slightly moveable joint. The joint contributes to pelvic stability and is strengthened by the tough and resilient set of anterior, posterior, and lateral sacrococcygeal ligaments.¹Protzer L, Seligson D, Doursounian L. The coccyx in clinical medicine. Current Orthopaedic Practice. 2017;28(3):314-317. DOI: 10.1097/BCO.0000000000000502

Sacrococcygeal disc

A thin fibro-cartilaginous disc is situated between S5 and Co1, and it serves as a cushion to absorb weight from the upper body.⁴Chaudhry SR, Nahian A, Chaudhry K. Anatomy, Abdomen and Pelvis, Pelvis. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022. https://europepmc.org/article/NBK/nbk482258 This disc is thinner and less robust than the other vertebral discs in the spinal column.⁴Chaudhry SR, Nahian A, Chaudhry K. Anatomy, Abdomen and Pelvis, Pelvis. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022. https://europepmc.org/article/NBK/nbk482258

Like other spinal discs, the sacrococcygeal disc may degenerate due to natural aging. However, it usually does not encounter herniation as it is exposed to lower mechanical stress levels than the other spinal discs.⁵Sandrasegaram N, Gupta R, Baloch M. Diagnosis and management of sacrococcygeal pain. BJA Educ. 2020;20(3):74-79. doi:10.1016/j.bjae.2019.11.004

Motion at the sacrococcygeal symphysis

The movement of the sacrococcygeal symphysis is limited to bending forward and backward. The joint helps in spinal flexion and extension, such as moving from standing to sitting.⁵Sandrasegaram N, Gupta R, Baloch M. Diagnosis and management of sacrococcygeal pain. BJA Educ. 2020;20(3):74-79. doi:10.1016/j.bjae.2019.11.004

While sitting, the coccyx moves slightly forward, allowing the body’s weight to be distributed evenly.⁴Chaudhry SR, Nahian A, Chaudhry K. Anatomy, Abdomen and Pelvis, Pelvis. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022. https://europepmc.org/article/NBK/nbk482258

During childbirth, the coccyx moves backward, increasing the diameter of the pelvic passage to aid in the delivery of a child.⁴Chaudhry SR, Nahian A, Chaudhry K. Anatomy, Abdomen and Pelvis, Pelvis. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022. https://europepmc.org/article/NBK/nbk482258 In some cases, the mother’s coccyx may fracture during labor and delivery.

Key Differences Between Coccygeal Vertebrae and Other Spinal Vertebrae

The coccygeal vertebrae and spinal vertebrae have some similarities, but they also have significant differences. 

• Each bone in the coccyx is somewhat smaller in both height and width compared to the bone above it. 
• The coccyx does not have an opening for the vertebral column; hence, there are no neural elements, such as the cauda equina or cerebrospinal fluid, in the coccygeal spine. ⁶Mostafa E, Varacallo M. Anatomy, Back, Coccygeal Vertebrae. [Updated 2022 Jun 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK549870/

Only the first and largest coccygeal vertebra, Co1, resemble other spinal vertebrae because of its transverse and superior articular processes.⁶Mostafa E, Varacallo M. Anatomy, Back, Coccygeal Vertebrae. [Updated 2022 Jun 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK549870/

Anatomical Variations of the Coccyx

Anatomical variations of the coccyx are common.Common variations reported in the literature include:

• Differences in the number of segments. According to the literature, around 76% of individuals have four coccygeal vertebrae, but some can have as few as three (13%) or as many as five (11%).⁷Woon JT, Perumal V, Maigne JY, Stringer MD. CT morphology and morphometry of the normal adult coccyx. Eur Spine J. 2013;22(4):863-870. doi:10.1007/s00586-012-2595-2
  Rarely, some individuals may have only two fused segments (hypoplastic coccyx), while others may have more than five segments due to an extra coccygeal vertebra (supernumerary vertebra).⁸Paudyal S, Karna BP, Joshi BR. Evaluation of Anatomical Variation of Human Coccyx on Magnetic Resonance Imaging. Nepalese J Radiol. 2022;12(2):2-6. doi:10.3126/njr.v12i2.54582.
• Changes in the degree of curvature. The curvature of the coccyx generally follows a slight forward curve, but some individuals may have a more exaggerated forward curve or have a straighter or even a slightly backward-curving coccyx.³Woon JTK, Stringer MD. Clinical anatomy of the coccyx: A systematic review. Clinical Anatomy. 2012;25(12):158-167. https://doi.org/10.1002/ca.21216 The average coccygeal curve length in males and females combined is 4.3 cm, and the mean coccygeal angle is 143°.³Woon JTK, Stringer MD. Clinical anatomy of the coccyx: A systematic review. Clinical Anatomy. 2012;25(12):158-167. https://doi.org/10.1002/ca.21216
• The presence of bony projections. While less common, some individuals may have bony projections or spurs on the coccyx.²Foye PM. Coccydynia: Tailbone Pain. Phys Med Rehabil Clin N Am. 2017;28(3):539-549. doi:10.1016/j.pmr.2017.03.006
• Atypical fusion of the coccygeal vertebrae. Some people may have only partial fusions between the coccygeal segments.⁸Paudyal S, Karna BP, Joshi BR. Evaluation of Anatomical Variation of Human Coccyx on Magnetic Resonance Imaging. Nepalese J Radiol. 2022;12(2):2-6. doi:10.3126/njr.v12i2.54582.
• The absence of the coccyx. In rare cases, some individuals may not develop the coccyx. This condition is called coccygeal agenesis.⁸Paudyal S, Karna BP, Joshi BR. Evaluation of Anatomical Variation of Human Coccyx on Magnetic Resonance Imaging. Nepalese J Radiol. 2022;12(2):2-6. doi:10.3126/njr.v12i2.54582.^,9Lee JY, Pang D, Wang KC. Caudal Agenesis and Associated Spinal Cord Malformations. In: Di Rocco C, Pang D, Rutka J, eds. Textbook of Pediatric Neurosurgery. Springer; 2020. https://doi.org/10.1007/978-3-319-72168-2_119

It is important to note that while anatomical variations in the coccyx are relatively common, they typically do not cause significant functional issues. However, in some cases, certain variations or abnormalities like coccygeal agenesis or bony projection may contribute to discomfort, pain, or other coccyx-related conditions.²Foye PM. Coccydynia: Tailbone Pain. Phys Med Rehabil Clin N Am. 2017;28(3):539-549. doi:10.1016/j.pmr.2017.03.006^,9Lee JY, Pang D, Wang KC. Caudal Agenesis and Associated Spinal Cord Malformations. In: Di Rocco C, Pang D, Rutka J, eds. Textbook of Pediatric Neurosurgery. Springer; 2020. https://doi.org/10.1007/978-3-319-72168-2_119

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